The Manhattan district attorney’s office is preparing state criminal charges against Paul J. Manafort, President Trump’s former campaign chairman, in an effort to ensure he will still face prison time even if the president pardons him for his federal crimes, according to several people with knowledge of the matter.

Mr. Manafort is scheduled to be sentenced next month for convictions in two federal cases brought by Special Counsel Robert S. Mueller III. He faces up to 25 years in prison for tax and bank fraud and additional time for conspiracy counts in a related case. It could effectively be a life sentence for Mr. Manafort, who turns 70 in April.

The president has broad power to issue pardons for federal crimes, but no such authority in state cases. And while there has been no clear indication that Mr. Trump intends to pardon Mr. Manafort, the president has spoken repeatedly of his pardon power and defended his former campaign chairman on a number of occasions, calling him a “brave man.”

The office of the Manhattan district attorney, Cyrus R. Vance Jr., first began investigating Mr. Manafort in 2017 in connection with loans he received from two banks. Those loans were also the subject of some of the counts in the federal indictment that led to his conviction last year. But the state prosecutors deferred their inquiry in order not to interfere with Mr. Mueller’s investigation into Russian interference in the 2016 presidential election.

They resumed their investigation in recent months, and a state grand jury began hearing evidence in the case, several people with knowledge of the matter said. The panel is expected to wrap up its work in the coming weeks, several of the people said, and prosecutors likely will ask the grand jurors to vote on charges shortly thereafter.

Mr. Vance’s office is expected to seek charges whether or not the president pardons Mr. Manafort. The plan was first reported by Bloomberg.

Any charges brought by Mr. Vance’s office would likely be challenged on double jeopardy grounds. New York state law includes stronger protections than those provided by the United States Constitution, and Mr. Manafort’s defense team is likely to challenge any state charges. But prosecutors in Mr. Vance’s office have expressed confidence that they would prevail, people with knowledge of the matter said.

Jason Maloni, a spokesman for Mr. Manafort, said his legal team had no comment. Mr. Vance’s office also had no comment.

Mr. Manafort, who worked for Mr. Trump’s campaign during a critical five months when he became the Republican Party’s presidential nominee in 2016, was convicted in federal court in Virginia in August on eight counts of various financial crimes. Prosecutors said Mr. Manafort used foreign accounts to hide millions of dollars from his political consulting work in Ukraine and evade taxes, and lied to banks to obtain millions of dollars in loans.

Weeks later, he agreed to plead guilty in a related case in federal court in Washington, D.C., and cooperate with prosecutors from Mr. Mueller’s office. But the deal blew up when a judge ruled he had repeatedly lied to the government about his contact with a Russian associate during the campaign and after the election. Prosecutors claim that the associate, Konstantin V. Kilimnik, has ties to Russian intelligence, and have been investigating whether he was involved in a covert attempt to influence the election results.

In the Manhattan case, the evidence presented to a grand jury appears to have been connected to loans issued by Citizens Bank in Rhode Island and Federal Savings Bank in Chicago.

The banks have received grand jury subpoenas for records relating to the loans they issued to Mr. Manafort, which were worth millions of dollars, people with knowledge of the matter said. The grand jury has also been hearing testimony about the loans. Citizens Bank has been cooperating with the investigation, according to a person with knowledge of the matter. A spokeswoman for Federal Savings Bank did not respond to a request for comment.

It is unclear precisely what charges Mr. Manafort would face, but they could include two state felonies: falsifying business records, if the evidence shows Mr. Manafort used the loan money for an unauthorized purpose, and mortgage fraud.

Correction:Feb. 22, 2019

An earlier version of this article misstated the number of counts Paul J. Manafort was convicted on. It was eight counts, not 10.